1. Field of the Invention
The invention relates to methods for depositing high quality films of complex (compound) materials on substrates at high deposition rates, and apparatus for effecting such methods. Particularly, the invention relates to enhanced chemical vapor deposition from liquid sources depositing high quality thin films of a large variety of complex compounds at high deposition rates, and apparatus for effecting such methods. More particularly the invention relates to apparatus and processes for fabricating high quality thin films of layered superlattice compounds.
2. Statement of the Problem
There are known methods for depositing thin films of complex compounds, such as metal oxides, ferroelectrics, super-conductors, materials with high dielectric constants, gems, etc. Such known methods include RF sputtering, chemical vapor deposition (CVD), and spin coating. RF sputtering does not provide thin films of suitably high quality for practical integrated circuit uses, and it is hard if not impossible to control the stoichiometry so as to produce materials within the strict requirements of integrated circuit uses. Spin coating avoids the above defects of sputtering, but does not have good step coverage and suitably high fabrication rates for commercial uses. Present methods of chemical vapor deposition, while having good step coverage, are simply not able to form complex materials of suitable quality for integrated circuit use. The application of known CVD methods to complex materials, such as layered superlattice compounds, results in premature decomposition of the reagents and, often, a dry dust, rather than a solid material deposited on the substrate, or result in inferior quality materials that are not suitable for use as active components in an integrated circuit.
It has been recently discovered that certain layered compounds, referred to herein as layered superlattice compounds, are far better suited for use in ferroelectric and high dielectric constant memories than any prior art materials. These materials are highly complex, and no method is available to reliably fabricate high quality layered superlattice compounds in commercial quantities, at high deposition rates, and with step coverage that is suitable for making state-of-the-art integrated circuits.
3. Solution to the Problem
The invention solves the above problems by providing methods and apparatus for the chemical vapor deposition of thin films of layered superlattice materials that avoid premature decomposition of the reagents, provide easily controlled composition and flow rate of a gas phase reactant stream to the CVD reactor, and result in a thin film containing small grains having mixed orientation and good electrical properties.
One aspect of the invention is a two-step vaporization process, comprising production of a mist of each liquid precursor, followed by rapid, low-temperature vaporization of the mist.
Another aspect of the invention is the use of polyalkoxide precursors containing a plurality of chemical constituents of the desired thin film in order to reduce the total number of liquid precursors.
The invention is useful for fabrication of thin films of ceramics, glasseous materials, electrically-active materials, including ferroelectric materials and high dielectric constant materials from liquid sources including sol-gel or MOD formulations. The invention in particular provides a method of fabricating an integrated circuit having at least one layered superlattice thin film deposited by the CVD process. Preferably the integrated circuit is a non-volatile memory.
The invention provides a method of making CVD precursors utilizing methoxides, ethoxides, butoxides, propoxides and other compounds with which CVD precursors may be made for almost any layered superlattice material.
Preferably the invention uses a metal polyalkoxide reagent. Compared with other metalorganics used in CVD, the polyalkoxides have a high decomposition temperature.
Preferably there is only one metal oxide precursor and one polyalkoxide precursor. Preferably, the metal oxide precursor contains a bismuth-containing oxide that reacts with a double alkoxide precursor to produce a bismuth-layered superlattice. The preferred method provides for mixing all precursors used in the CVD process in a common solvent, such as tetrahydrofuran, prior to the misting step. In an alternative method, the invention provides for forming a mist of each liquid precursor separately, and then combining the separate mist streams for vaporization.
The invention provides an apparatus for forming a mist, preferably comprising a venturi-mister. The venturi-mister produces a mist of variable, controllable mass flow rates, comprising droplets of narrow, controllable size distribution. Because the mass flow rate and chemical composition of the mist is known, it is possible to deposit a thin film of uniform, desired composition and stoichiometry.
The invention provides for flowing the mist process streams through tubing at ambient temperature, thus avoiding premature decomposition of reagents at elevated temperature in the process tubing.
The invention provides for a heating zone in which the liquid reagents contained in the mist droplets in the flowing mist stream are gasified quickly at low temperature, i.e. before entering a deposition zone, thus avoiding premature decomposition at elevated temperature during gasification. This is possible because the droplets, being small, have a large surface to volume ratio and are moving at a finite velocity in the carrier gas through the heating zone. The resulting enhanced heat transfer rate enables the latent heat of vaporization necessary to gasify the liquid droplets in the mist to transfer to the liquid at a temperature below the ranges in which premature decomposition of the reagents could occur. Also, the misting and subsequent low-temperature gasification of precursor liquids enables the use of low-volatility reagent compounds, which otherwise would not be usable in a CVD process because of their low volatility.
The invention provides for decomposition of vaporized reagents and formation of the integrated circuit thin film, particularly a layered superlattice compound thin film, on a substrate in the deposition reactor at between 300.degree. C. and 500.degree. C. This produces an amorphous or polycrystalline phase with relatively small grain boundaries.
The invention provides for ion-coupled plasma (ICP) excitation of the reactant gas in the deposition reactor, which accelerates the rate of decomposition and reaction by overcoming kinetic barriers to reaction without adding heat to the reaction.
An alternative of the invention provides for a lead-containing organic precursor in order to produce a Pb-containing Bi-layered superlattice compound.
Another alternative provides for UV-irradiation of the reactant gas in the deposition reactor to enhance reagent decomposition and electronic properties of the deposited thin film.
The invention also optionally provides for an ion implantation step after the formation of the layered superlattice thin film and prior to the annealing step. This ion implantation step creates ion damage on the surface which provides a large number of crystallization nucleation points of different orientations.
The invention provides for a crystallization or recrystallization step after the thin film is formed in the CVD process. Preferably the thin film is crystallized or recrystallized in a furnace anneal at a temperature of from 400.degree. C. to 900.degree. C., preferably 750.degree. C. A rapid thermal processing (RTP) anneal may also be used. In the RTP anneal the temperature is ramped over a range of from 1.degree. C. per second to 300.degree. C. per second and up to a temperature of from 500.degree. C. to 850.degree. C. for a holding period of from 5 seconds to 5 minutes.
The invention provides for depositing an electrode or contact on the material, such as a layered superlattice material, followed by a second anneal, preferably a furnace anneal at from 600.degree. C. to 850.degree. C. for a period of 15 minutes or more.
Preferably each of the heating steps, i.e. the CVD process, the first anneal and the second anneal after contact formation takes place at the same or a higher temperature than the preceding heating step.
Preferably the invention also includes a step of prebaking the substrate in an oxygen furnace at a temperature of between 500.degree. C. and 1000.degree. C. prior to performing the CVD deposition step.
The methods described above result in layered superlattice materials with good electronic properties, such as high polarizability, high dielectric constants, and low leakage currents. This is believed to be due to a crystalline orientation that results in good electronic properties.